Electron discharge device amplifier



March 26, 4 o. E. KEALL 2 ,1 ,55

ELECTRON DISCHARGE D VICE AMPLIFIER Fil ed Jfine 5, 1956 Flefiaz/mcy cYczEs PER sscolvp db A l i 4 I I [I I 5 INVENTOR .0/ I0 I00 OSWOLD EDWARD KEALL FREQUENCY, CYCLE-S PER SECOND BY I ATTORNEY Patented Mar. 26, 1940 PATENT OFFICE ELECTRON DISCHARGE DEVICE AMPLIFIER Oswold Edward Keall, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application June 6, 1936, Serial No. 83,867 In Great Britain June 21, 1935 5 Claims. 01. 179-171) This invention relates to electron discharge de-, vice amplifiers suitable for use for ampifying very low frequencies for example for use in connection with electrocardiographs and like appa- 5 ratus for recording or audibly reproducing heart beats.

Where amplificationof very low frequencies such" as heart beat frequencies is required and resistance capacity coupled thermionic valve amplifiers are sought to be used for this purpose the time constants of the circuit must be selected very large and this involves that the amplifier becomes both uneconomical and bulky largely owing to the relatively great size of the condensers required. It is well known that relatively distortionless amplication of very low frequencies and slow transients can be obtained by means of direct coupled amplifiers but the said amplifiers are not easily adapted to mains operation since the potentiometer necessary for selecting the grid pacity coupled amplifiers may be used so long as their time constant corresponds to the period of repetition. If this period is say 5 seconds, the

time constant, which is, of course, equal to the 30 product of the resistance and the capacity of the circuit in question, must beequal to 5. In a resistance-capacity coupled amplifier a limit;is usually set to the value of resistance which can be used in the output circuit of any stage by the resistance necessarily present in the input circuit of. the succeeding stage this resistance being rarely larger than 2 megohms. Accordingly the resistance in the output circuit of a preceding stagein a resistance capacity coupled amplifier can rarely be allowed to exceed half a megohm and for a 5 second time constant this involves the use of a condenser of 10 microfarads-a very large condenser. It is, of course, possible to increase the effective time constant of a stage 45 by the use of de-coupling condensers and resistances,.but the provision of such means involves an increase in the voltage of the source from the above mentioned difiiculties and practical disadvantageaand according to the said invention, there is included in the grid circuit of a 55 stag'e'of resistance capacity coupled amplification an additional circuit including resistance and capacity in parallel, this additional circuit being in series with the normally provided grid resistance. It should be noted that an essential feature of the invention and one that is very im- .5 portant in practice, is that the additional circuit is provided in a grid circuit as distinct from in an anode circuit for by reason of the location of said additional circuit in this manner any necessity for increase of the potential of the w anode potential source is obviated. Were the additional circuit in an anode circuit, the voltage drop thereacross would have tobelmade good by the anode potential source, and as above stated this is in practice undesirable. it

The invention is illustrated in and further explained in connection with the accompanying diagrammatic and graphical drawing of which Fig. 1 shows the circuit of the invention and Figs. 2 and 3 show. curves illustrating certain go characteristics of the circuit.

Referring to Figure 1 which shows one way of carrying out this invention as applied to a two stage resistance capacity coupled amplifier suitable for use for amplifying slow transients or low frequencies, such as heart beats, input voltage tobe amplified is applied at terminals 1 between grid 2 and cathode 3 of a triode 4 whose cathode lead contains the usual automatic bias resistance arrangement represented by the rectangle 5 and whose anode receives operating potential from a source (not shown) through the usual anode resistance 6. In addition the anode-cathodecircult of the valve 4 includes in the order stated and commencing at the anode a coupling conthe cathode 3 through the bias resistance arrangement 5. The portion of the anode circuit consisting of the elements 8, 9 and I0 is also connected between the grid l l and the cathode [2 of the valve I3 in the next stage. Withthis circuit arrangement it is possible to choose values of the components 1, 8, 9 and H) in relation to the anode'load of valve 4 in such manner that a substantially rectilinear frequency response may be obtained down to a predetermined low frequency, for example, .1 of a cycle per second, and the coupling resistance 8 and coupling condenser may be chosen to have a time constant which is 10 times less than that which they would" have were the additional circuit not provided and the said time constant accordingly selected to correspond to the frequency of .1 of a cycle per second.

In a practical example as illustrated in Figure 1 and designed to amplify down to a frequency of .1 of a cycle per second, the coupling condenser I was of 2 microfarads, thecoupling resistance 8 was of 100,000 ohms, the condenser ID was of l microfarad, the resistance 9 was of 1 megohm, the anode resistance 6 was of 250,000 ohms and the valve 4 was a high impedance triode with an effective impedance of about 100,000 ohms.

It is worthy of note that with the above described circuit arrangement, the results are not achieved at the expense of phase shift. In the particular example above referred to, with the additional circuit provided the phase shift remained between about :5" down to a frequency of about .35 of a cycle per second, the phase shift thereafter rising to about +25 at .1 of a cycle per second and to about +45 at .05 of a cycle per second. These results are shown graphically by the curve A in Figure 2 phase shift in degrees being plotted as ordinated against frequency. For the same circuit but with the additional circuit omitted the phase shift was about +45 at .8 of a cycle per second, dropping as the frequency rose but not falling as low as +5 until the frequency was about cycles per second, the fall being roughly rectilinear. This result is shown graphically by the broken line curve B in Figure 2. The curve C in Figure 2 shows the results obtained with the condenser ll of 0.1 microfarad, the circuit details being otherwise as for curve A.

As regards frequency response characteristic, with the specifically described embodiment the gain in decibels was within the limits $1.5 down to .06 of a cycie per second, and below this frequency the gain dropped off sharply giving a low frequency cut ofi effect. This is shown graphically by curve D in Figure 3 in which gain in decibels (ordinates) is plotted against frequency. Withthe same circuit modified however by the omission of the additional circuit,

1 the gain was 22 at .06 of a cycle per second, and

rose approximately rectilinearly to about -4 at .7 of a cycle per second, the curve thereafter flattening cut. Part of this result is represented by the broken line curve E in Figure 3. With the specifically described embodiment modified by using, in place of a condenser S0 of 1 microfarad. a condenser ll: of 0.1 microfarad. a curve as shown at in Figure 3 was obtained. This curve shows a rising characteristic as frequency is reduced from '70 to 7 cycles per second, the curve then remaining level at about +4 decibels down to .2 of a cycle per second, thereafter falling approximately rectilinearly to about -6 decibel at .02 of a cycle per second. From the corresponding phase shift curve C of Figure 2 it will be seen that phase shift remains between il2 down to .3 of a cycle per second thereafter rising approximately rectilinear-1y to +-i5 at about .065 of a cycle'per second. Such an arrangement could be utilized to give increased response at low frequencies or to correct for a loss in low frequency response in some other circuit. It will be appreciated that in order to obtain a large amount of correction the grid coupling resistance should be small as compared to the anode load resistance while the resistance of the additional circuit should be of the same order as, or higher than the anode load resistance. The valves of the grid coupling condenser and the condenser in the additional circuit may then be chosen to produce a desired correction in a desired part of the frequency spectrum to be handled by the amplifier.

Although the invention has been particularly described in respect to its application to cardiograph amplifiers, it is not limited to such application and may be applied to all amplifiers for operation at very low frequencies. As will now be seen another application which is rendered possible by the nature of the frequency response curves obtainable, is to amplifiers required to cut off sharply at a predetermined low frequency and heavily attenuated frequencies below the said predetermined low frequency. The invention is accordingly applicable to high pass or band pass filters and is very advantageous when so applied as the phase shift remains approximately constant and at a low value at frequencies above the cut off value.

Where the invention is applied to cases where the low frequency cut off effect is required to be made sharp the said out off effect may be sharpened up by the provision of an inductive element 5 i in parallel with the resistance and capacity in the additional circuit. In this connection it will be appreciated that since the circuit will be effective in the manner desired only when the dynamic resistance of the inductance and condenser in the additional circuit approaches the value of the resistance in said circuit, practical considerations will limit the application of embodiments involving an inductance in the additional circuit to amplifiers operating at frequencies considerably higher than the extremely low frequencies nm'nerically specified hereinbefore. Nevertheless the modified circuit wherein an inductance is inciuded in the additional circuit is of fairly wide application, and it will be found of considerable advantage in relatively low frequency amplifiers where a sharp low frequency cut off is required and the obtaining of an approximately constant small phase shift at frequencies above this cut off point is desired.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed I declare that what I claim is:

1. In a resistance capacity coupled low frequency amplifier adapted to provide substantially rectilinear frequency response down to one tenth of a cycle per second, a first tube having an anode. a cathode and a control electrode and a second tube provided with an anode, a cathode and a control electrode, means for impressing the low frequency signal energy between the grid electrode and cathode of the first tube, means including a resistor element of the order of 250,000 ohms for connecting the anode of the first tube to a source of space current, a coupling condenser of the order of two microfarads directly connected between the anode of the first tube and the grid electrode of the second tube, a connection between the cathode of the first tube and the cathode of the second tube, a grid resistor for said second tube and an additional resistor in series connected between the grid of the second tube and the cathode thereof, said grid resistor being of the order of 100,000 ohms and said additional resistor being of the order of l megohm and a condenser shunted across the additional resistor, said condenser being of the order of l microfarad.

2. In a resistance capacity coupled low frequency amplifier adapted to provide substantially rectilinear frequency response down to a fraction of acycle per second, a first tube having an anode, a cathode and a grid electrode, and a second tube having anode, cathode and grid electrodes, means for impressing the low frequency energy to be amplified upon the grid electrode of the first tube, a grid circuit for the second tubeincluding a grid resistor in series parallel arrangement not provided whereby frequencies as low as a fraction of a cycle per second maybe amplified in said amplifier.

3. In a resistance capacity coupled, low frequency amplifier having a sharply defined low frequencycut-ofi point, a first tube having an anode, a cathode and a grid electrode, a second tube provided with an anode, a cathode and a grid electrode,,means for impressing the low frequency energy upon the grid electrode of the first tube, a grid circuit for the second tube includ-' 1 ing a grid resistor and in series therewith a parallel arrangement of a resistance and condenser,

a coupling condenser connected between the anode of the first tube and the grid electrode of the second and means comprising an inductance element connected in parallel with the last named resistance for determining the low frequency cut-off point of the amplifier.

4. In an amplifier circuit, a first electronic tube provided with an anode, a cathode and a grid electrode, external circuitsrelated to said tube electrodes and forming an input circuit and,

, an output circuit forsaid tube, said tube producing amplified voltage in its output circuit from voltage impressed upon its input circuit,

a second electronic tube having an input electrode, means for impressing the output of said c first tube upon the input electrode of said second tube comprising a voltagedivider connected between the anode and cathodeof said first tube, said voltage divider comprising two impedance networks connected in series, said input electrode being connected to the junction of said two impedance networks, the first of said impedance networks comprising a series condenser which causes the impedance of the first network 7 to increase with decreasing frequency thereby tending to reduce the fraction of the original voltage thatis fed to theinput electrode of the second tube, the second of said two impedance,

networks comprising a series resistance and in series therewith a combination of impedances permeable to direct current and providing an as the frequency decreases over a useful range to such an extent that the voltage division produced by said divider is rendered more uniform over said range of useful frequencies.

\ 5. An arrangement as described in claim 4 wherein said second of said two impedance networks comprises a resistor in series with a second resistor shunted by a condenser.

OSWOLD EDWARD KEALL.

H25 impedance to alternating current which increases 

